Fuel assemblies must be checked systematically in several situations. Assemblies already irradiated but not exhausted must be checked before reloading them into the core during a refueling operation. Fully spent assemblies which have been left for some time in a deactivation pool must be checked before they are sent to a reprocessing plant. Additional precautions must be taken during reprocessing and during dismantling of assemblies having leaky fuel elements.
The most frequently used method for testing the tightness of the sheaths is sipping, consisting in increasing the pressure of the gases in the sheaths by increasing the temperature (by supplying heat or by the residual thermal power of the fuel) so that fission products are released in case of a defect, and measuring the activity or increase of activity of a fluid sweeping the sheaths of the inspected assembly.
Different fission products escape through possible cracks. They may comprise gases, such as xenon 133, and water soluble products such as iodine 131. When the assembly being tested has deactivated, the gaseous fission products are not very abundant or have parameters which are difficult to measure in line. The analysis of a gas flow which has swept the assembly then provides unmeaningful results. On the other hand, such assemblies will contain water soluble fission products (iodine 131 after a short cooling period and isotopes 134 and 137 of caesium for longer cooling periods), and monitoring a water flow which has swept the assembly will allow defect detection. Conversely, such a water flow will not allow direct detection of fission products revealing a defective sheath in an assembly removed from a reactor at an early stage and immediately subjected to the test. Monitoring a gas flow allows meaningful results to be obtained.
In a prior art process (French Pat. No. 2,389,202) for testing the tightness of the cladding sheaths of a nuclear fuel assembly, the assembly is placed in a hood immersed in a protective pool, the assembly is heated to a predetermined temperature, a fluid is flowed through the chamber for entraining the gaseous fission products possibly released through defects, and the activity of the fluid leaving the hood is measured. According to that patent, the hood comprises heating to the required temperature plates for rapidly heating those of the assemblies which have only a low residual thermal power. The device further comprises means for carrying out the test either by aspiration of water having swept the assembly or by extraction of bubbling gas.